fix(modbus): review r0 - FC23 opt-in, explicit maxReadGap 0 = off, legacy coil types load, one health writer

Coordinator rulings and Important findings from review r0:
1. FC23 is opt-in (new UseFC23ForMultiRegisterWrites, default false); a config
   without it - including the persisted "disableFC23": false every form-saved
   config carries - keeps the FC16 wire. DisableFC23 stays the hard kill-switch.
2. An explicit maxReadGap 0 means coalescing off again (its pre-lane meaning);
   the gap-8 default applies only to an absent key.
3. Coils/DiscreteInputs tags persisted with a register dataType (the old
   editor saved Int16) are coerced to Bool, never refused, with no deploy
   warning (a warning can be promoted to a blocking error). The editor loads
   them as Bool. Register region + Bool is still refused.
4. One comms-fault slot for reads and writes: a good read clears a write's
   transport fault and vice versa.
Minors: Dispose writes its own Unknown health (m1); persisted region/address
are authoritative over addressString so a Family change cannot remap a live
tag (m2); a malformed-but-delivered reply no longer fails the unit fast (m3);
a fresh socket published after a concurrent dispose is torn down (m4); the
editor resets address text per tag (m5); master merged (m6); the FC23 latch
log says re-init restores it (m7). Back-compat tests use the old persisted
JSON shapes and fail without the fixes.

Refs #741
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus.Tests/ModbusCoalescingTests.cs

@@ -42,9 +42,9 @@ public sealed class ModbusCoalescingTests
         public ValueTask DisposeAsync() => ValueTask.CompletedTask;
     }
 
-    /// <summary>MaxReadGap=0 coalesces only adjacent tags: a one-register hole splits the read.</summary>
+    /// <summary>MaxReadGap=0 means no coalescing (its historical meaning, kept for persisted configs).</summary>
     [Fact]
-    public async Task MaxReadGap_Zero_Does_Not_Bridge_A_Gap()
+    public async Task MaxReadGap_Zero_Means_No_Coalescing()
     {
         var fake = new CountingTransport();
         var t1 = new ModbusTagDefinition("T1", ModbusRegion.HoldingRegisters, 100, ModbusDataType.Int16);
@@ -56,7 +56,7 @@ public sealed class ModbusCoalescingTests
 
         await drv.ReadAsync(["T1", "T2"], CancellationToken.None);
 
-        // A one-register gap with MaxReadGap=0: expect 2 separate FC03 reads.
+        // MaxReadGap=0 is coalescing OFF: expect 2 separate FC03 reads.
         var fc03Reads = fake.Reads.Where(r => r.Fc == 0x03).ToList();
         fc03Reads.Count.ShouldBe(2);
     }